There are a number of systems like the one used for Joachim Sauter's
"Zehrseher"
(http://www.artcom.de/projects/zerseher/welcome.en). The beauty of it's
interface
was the lack thereof. When a visitor looked at an image, a computer looked
at the
visitor, tried to pinpoint the viewer's eyes, and modified the image in
real time -- a
very complex task which required extremely powerful computers and continual
recalibration.

Many other people have developed head-mounted systems for eye-tracking. A
couple of these even run through MM Director on a Mac.

I'm not certain what you want from your project, but you should realise
that these
systems are very intimate and personal. They are difficult to term
performance
systems. Ulrike and Joachim did remarkable jobs turning their pieces into
installations,
and allowing the rest of the room to participate. But in an installation,
the focus of the performance shifts to an audience watching a watcher. It's
akin to a magician calling
somebody up on stage to help with a card-trick.

Reading your mis-directed email, I wonder how an audience will perceive all
these
wondrous interactive elements you intend to toss together. What is the
difference
between an interactive, choreographed performance, and dancers taking cues
from a video tape?

I know it's an absurd question from somebody whose interactive performance
is touring
Europe as a videotape.